The present invention relates generally to sensors, and more particularly to ferromagnetic semiconductor-based sensing devices and methods.
Ferromagnetic semiconductors are of considerable current interest since they offer prospects for realizing semiconducting spintronics devices that have no analogs in a metallic ferromagnetic system (see, e.g., G. Prinz, Science 282, 1660 (1998); S. A. Wolf et al., Science 294, 1488 (2001).). One recent and striking example is the electric field control of ferromagnetism (see, H. Ohno et al., Nature (London) 408, 944 (2000).). Semiconductor-based magnetic materials also offer new possibilities for attaining great improvements in performance over metallic magnetic devices. Among the examples here is conductivity matching to attain efficient spin injection into semiconductors (see, e.g., G. Schmidt et al., Phys. Rev. B 62, R4790 (2000).). Semiconductor ferromagnetism also gives rise to new physical phenomena because it is possible to engineer, and enhance, spin-orbit coupling in ways that are not possible in metallic systems.
Advances in nanofabrication and epitaxial growth bring new levels of control and resolution to the study of magnetic domains at the microscopic level. Recent research efforts are focused upon two principal thrusts: studies of domain wall dynamics, and investigations of domain wall resistance. After decades of concerted study on metallic ferromagnetic thin films, it is generally agreed that magnetization reversal within high quality magnetic microstructures occurs first by nucleation, then propagation, of domain walls (see, e.g., J. Ferré, Topics Appl. Phys. 83, 127 (2002)). As regards the latter, even though the resistance of domain walls is difficult to measure, it has become a topic of significant current interest (see, M. Viret, et al., Phys. Rev. Lett. 85, 3962 (2000); U. Ruediger, J. Yu, S. Zhang, A. D. Kent, S. S. P. Parkin, Phys. Rev. Lett. 80, 5639 (1998); L. Klein et al., Phys. Rev. Lett. 84, 6090 (2000); R. Danneau et al., Phys. Rev. Lett. 88, 157201 (2002); U. Ebels, A. Radulescu, Y. Henry, L. Piraux, K. Ounadjela, Phys. Rev. Lett. 84, 983 (2000); T. Taniyama, I. Nakatani, T. Namikawa, Y. Yamazaki, Phys. Rev. Lett. 82, 2780 (1999); and Y. B. Xu et al., Phys. Rev. B 61, R14901 (2000)). This is due to both the important role domain wall resistance might play in advanced spintronic devices, as well as the challenges it poses for both experimental and theoretical attempts to obtain fundamental understanding of its underlying physics.
Currently, there are several experimental approaches to the study of domain wall dynamics. One of the principal techniques is time-resolved imaging via the magneto-optic Kerr effect (MOKE) (see, S. B. Choe, S. C. Shin, Phys. Rev. Lett. 86, 532 (2001); S. Lemerle et al., Phys. Rev. Lett. 80, 849 (1998); and D. A. Allwood et al., Science 296, 2003 (2002)), an optical method that can provide sufficient contrast to directly observe the growth or shrinkage of magnetic domains within a sample. Another recently developed approach is based upon electrical measurements via the giant magnetoresistance (GMR) effect, which has enabled investigations of domain wall propagation in submicron Permalloy wires (see, e.g., T. Ono et al., Science 284, 468 (1999)). In the latter work, the observed ˜1 Ω GMR jumps emanate from regions that are smaller than is readily accessible to MOKE. However, this electrically-based approach is complicated by relatively small signal levels and the sample's multiple ferromagnetic layers.
At present, understanding of domain wall resistance is unsettled—there exists a conflicting body of work in the literature. Early theoretical studies (see, e.g., G. G. Cabrera, L. M. Falicov, Phys. Status Solidi B 61, 59 (1974); ibid. 62, 217 (1974) and L. Berger, J. Appl. Phys. 49, 2156 (1978)) predict that a domain wall should contribute minimally to the resistivity of a ferromagnet. This is predicated upon adiabatic electron transfer across a domain wall, based upon the reasonable assumption that the wall width is much greater than the Fermi wavelength. However, it was also pointed out that the different magnetization orientations of adjacent domains might locally perturb the current distribution and thereby yield an effect. More recent theoretical efforts strive to clarify this situation, but models yielding both positive and negative domain wall resistance have been explicated (see, P. M. Levy, S. Zhang, Phys. Rev. Lett. 79, 5110 (1997); G. Tatara, H. Fukuyama, Phys. Rev. Lett. 78, 3773 (1997); R. P. van Gorkom, A. Brataas, G. E. W. Bauer, Phys. Rev. Lett. 83, 4401 (1999); and M. Viret et al., Phys. Rev. B 53, 8464 (1996)). On the experimental side, measurements of domain wall resistance have been indirect; it is typically inferred by measuring the excess resistance arising when large ensembles of domain walls are nucleated within a sample (see, M. Viret, et al., Phys. Rev. Lett. 85, 3962 (2000); U. Ruediger, J. Yu, S. Zhang, A. D. Kent, S. S. P. Parkin, Phys. Rev. Lett. 80, 5639 (1998); and L. Klein et al., Phys. Rev. Lett. 84, 6090 (2000)). Recent progress in domain imaging and nanofabrication now allows resistance measurements on samples containing significantly reduced numbers of domain walls (see, U. Ebels, A. Radulescu, Y. Henry, L. Piraux, K. Ounadjela, Phys. Rev. Lett. 84, 983 (2000); T. Taniyama, I. Nakatani, T. Namikawa, Y. Yamazaki, Phys. Rev. Lett. 82, 2780 (1999); and Y. B. Xu et al., Phys. Rev. B 61, R14901 (2000)). However, to date, experimental results that have emerged from these various methods are also conflicting; both positive (see, M. Viret, et al., Phys. Rev. Lett. 85, 3962 (2000); L. Klein et al., Phys. Rev. Lett. 84, 6090 (2000); U. Ebels, A. Radulescu, Y. Henry, L. Piraux, K. Ounadjela, Phys. Rev. Lett. 84, 983 (2000); and Y. B. Xu et al., Phys. Rev. B 61, R14901 (2000)) and negative (see, U. Ruediger, J. Yu, S. Zhang, A. D. Kent, S. S. P. Parkin, Phys. Rev. Lett. 80, 5639 (1998) and T. Taniyama, I. Nakatani, T. Namikawa, Y. Yamazaki, Phys. Rev. Lett. 82, 2780 (1999)) domain wall resistance are reported.
In magnetic data storage technologies, and in particular magnetic hard disk drive (HDD) technologies, one goal is to increase storage capacity by, in part, reducing key dimensions within the HDD (“scaling”). Recently, megnetoresistive (MR) and giant magetoresistive (GMR) materials have been developed for use in read heads that help facilitate HDD scaling using MR and GMR materials with improved sensitivity to magnetic fields on a disk storage medium. Increased sensitivity makes it possible to detect smaller recorded bits and to read these bits at higher data rates. Larger signals from GMR heads also help reduce electronic noise. MR and GMR materials and devices however, are limited in sensitivity and therefore in application.
It is clear that there is a need for ferromagnetic semiconductor materials that provide enhanced magnetic sensitivity as well as ferromagnetic semiconductor-based sensing systems and methods that provide enhanced measurements using domain wall resistance and domain wall dynamics as well as magnetic switching effects.